ABSTRACT

The effect of pH of the rinsing agent on the content of B₂O₃, CaO and MgO in calcined magnesium oxide obtained from seawater by substoichiometric precipitation (where precipitation of magnesium hydroxide took place with 80% of the stoichiometric quantity of the dolomite lime as the precipitation agent) has been investigated.

The purpose of the study has been to ensure high purity magnesium oxide, particularly regarding the B₂O₃ content, because boron causes a problem in refractory magnesia for specialized refractory applications where high hot-strength is required.

It has been established that the content of B₂O₃ in magnesium oxide samples is significantly reduced when the pH value of the agent used for rinsing the magnesium hydroxide precipitate increases.

Keywords: Magnesium Oxide, Seawater, B₂O₃ content, Substoichiometric Precipitation, Rinsing Agent

1. INTRODUCTION

Although thermal decomposition of magnesite (MgCO₃) is the most frequent method of obtaining magnesium oxide [1-5], reserves of quality magnesite have been decreasing, so that chemical procedures for obtaining magnesium oxide [6-16], especially high purity one, have been studied all over the world for a number of years. One of these procedures is obtaining magnesium oxide from seawater.

Magnesium oxide obtained from seawater [9-16] is one of the most important materials used for production of high temperature resistant ceramic, and its advantage does not lie only in large reserves of seawater (1 m³ contains 0.945 kg of magnesium) but in higher purity of magnesium oxide sintered (above 98 mass %).

The basis of this technology is obtaining non-soluble magnesium hydroxide from seawater by precipitation with dolomite lime or lime from limestone, and by settling of the precipitate formed. The precipitate is washed and calcined to form caustic magnesia (MgO).

The settling rate is the "bottleneck" of this technology and it is one of the most important unit operations determining the cost of production. The settling rate can be significantly increased if small quantities of organic long chain polymers [15-18] are used, as well as if incomplete precipitation of magnesium hydroxide takes place. At precipitation of 80%, the capacity of the thickener increases by 86.5 % relative to complete precipitation [15].

In such a case, i.e. when this precipitation method is applied, the boron content adsorbed onto magnesium hydroxide during the precipitation process is somewhat higher than at stoichiometric precipitation, and should therefore be reduced. Boron is a particularly problematic impurity for magnesia to be used as high-quality refractory material. Thus, boron can be a problem in refractory magnesia for specialized refractory applications where high hot-strength is required.

The aim of this study was to examine the method of rinsing the precipitate of magnesium hydroxide obtained from seawater by substoichiometric 80 % precipitation in order to reduce the boron (B₂O₃) content in the final product, i.e. magnesium oxide.

2. EXPERIMENTAL

The seawater used for precipitation of magnesium hydroxide had the following content of magnesium oxide and calcium oxide: 2.1794 g dm^-3 MgO; 0.5557 g dm^-3 CaO.

The composition of the dolomite lime used was as follows (mass %): MgO = 40.90 %, CaO = 57.89 %, SiO₂ = 0.102 %, Fe₂O₃ = 0.319 % and Al₂O₃ = 0.866 %.

The seawater has first been pretreated to remove the bicarbonate and carbonate ions by adding a defined quantity of H₂SO₄ with on-line control through pH measurement (pH = 3.8 - 4.0), and degassing the acidified water to remove the released CO₂. Degassing was accomplished in a desorption tower by blowing air. The flow rate of the induced air was 120 dm³h^-1, and the volumetric flow rate of seawater through the desorption tower was 6 dm³ h^-1.

After pretreatment of seawater, precipitation of magnesium hydroxide took place with 80% of the stoichiometric quantity of dolomite lime. The precipitation reaction took 30 min., a magnetic stirrer was used. The sedimentation rate was increased by the addition of the Flocal-B (polyacrilamid) flocculant, in the optimum quantity. The experimental procedure to determine the optimum quantity of Flocal-B has been described in a previous investigation [15].

The precipitate obtained was then decanted and rinsed. The operating conditions were varied depending on the agent used for rinsing the magnesium hydroxide precipitate.

The rinsing agent was:

- distilled water, pH = 5.95

- alkalized distilled water, pH = 11.00 and pH = 12.50, which was alkalized by addition of concentrated NaOH.

The magnesium hydroxide precipitate obtained was rinsed by decanting and filtering. The rinsing and decanting procedure was repeated five times with approximately 1 dm³ of the rinsing agent. Then the magnesium hydroxide precipitate was filtered through a number of funnels. The rinsing agent used with magnesium hydroxide precipitate on the filter paper was the same as the one used for rinsing by decanting. The procedure was repeated five times, i.e. until rinsing was completed.

The magnesium hydroxide obtained was dried at 105^oC, and then calcined at 950^oC for 5 h to form caustic magnesia. The boron content in the sample examined was determined potentiometrically. The variation coefficient for the method applied is ± 1% [19].

The results listed represent average values of a series of measurements. (An average of five analyses in each case.)

3. RESULTS AND DISCUSSION

Examinations were carried out with magnesium oxide samples prepared by rinsing the precipitate with alkalized distilled water, non-alkalized distilled water and without previous rinsing of the magnesium hydroxide precipitate.

Table 1 shows operating conditions during rinsing of the magnesium hydroxide precipitate obtained from seawater by substoichiometric 80 % precipitation, as well as the experimentally obtained values for the composition of magnesium oxide samples.

Table 1. Chemical compositions (mass %) of magnesium oxide samples (80% precipitation)

after calcining at 950 ^oC / 5 h.

No. of samples	Rinsing agent	pH of rinsing agent	CaO	MgO	B	B₂O₃
No. of samples	Rinsing agent	pH of rinsing agent	mass %
1.	distilled water	5.95	0.85	98.62	0.0548	0.1764
2.	alkalized distilled water	11.00	0.95	97.89	0.0372	0.1198
3.	alkalized distilled water	12.50	1.28	97.83	0.0161	0.0518
4.	without rinsing		2.613	96.77	0.0602	0.1937

The experimental part of this study has examined the effect of the rinsing agent used for rinsing the precipitate of magnesium hydroxide obtained from seawater by stoichiometric 80 % precipitation. Examinations were carried out in order to reduce the boron (B₂O₃) content in the final product – magnesium oxide, because the hot-strength properties of certain magnesia refractory product are significantly affected by their boron content.

Boron occurs in seawater partly as non-dissociated orthoborate acid (H₃BO₃) and partly as borate ions (H₂BO₃^-), and during the magnesia precipitation process boron is adsorbed onto magnesia. The orthoborate acid is a weak acid with the following dissociation constants:

H₃BO₃ = H⁺ + H₂BO₃^-	K₁ = 5.8^.10^-10
H₂BO₃^- = H⁺ + HBO₃^2-	K₂ = 1.8^.10^-13
HBO₃^2- = H⁺ + BO₃^3-	K₃ = 1.6^.10^-14

The concentration of higher oxidation level ions HBO₃^2- and BO₃^3- is very low. By calculating the dissociation rate, one can establish the molal concentrations of H₂BO₃^-, HBO₃^2- and BO₃^3-, as well as the molal dissociation rate for every degree of dissociation of the orthoborate acid.

Fig. 1. shows concentrations of H₂BO₃^-, HBO₃^2- and BO₃^3- relative to pH of the rinsing solution (5.95, 11.00 and 12.50), as well as to pH of seawater during the precipitation process (9.6), while figure 2 shows the molal dissociation rate of H₃BO₃relative to pH.

Figure 1. The concentration of H₂BO₃^-, HBO₃^2- and BO₃^3- relative to pH

Figure 2. The molal dissociation rate of the orthoborate acid (H₃BO₃) relative to pH

For 80% precipitation of magnesium hydroxide from seawater by dolomite lime, the pH value is 9.6 during reaction precipitation and settling of the precipitate formed. In that case the orthoborate acid dissociation in the first degree is 69.78 %, which contributes to a significant increase of the B₂O₃ content in the product, i.e. in magnesium oxide obtained from seawater (0.1937 mass %).

Experimental data for chemical composition of magnesium oxide samples used indicate that the method of rinsing of magnesium hydroxide precipitate greatly affects the chemical composition of these samples, primarily as regards the content of CaO and B₂O₃in calcined magnesium oxide. Examinations show that when the magnesium hydroxide precipitate is rinsed with alkalized distilled water with pH = 12.50, the B₂O₃ content in the final product is greatly reduced.

Experimental results indicate that the MgO content in the sample prepared by rinsing the magnesium hydroxide precipitate with distilled water with pH = 5.95 amounts to 98.62 mass %, being slightly higher than the MgO content in the sample prepared by rinsing the magnesium hydroxide precipitate with alkalized distilled water with pH = 11.00 (MgO = 97.89 mass %), as well as with pH = 12.50 (MgO = 97.83 mass %). They differ for 0.8 %. In the magnesium oxide sample prepared without rinsing the magnesium hydroxide precipitate, the MgO content is markedly lower (96.77 mass %) due to higher impurity content.

Although examinations indicate that the sample contains a significant CaO content (0.95 mass %, and 1.28 mass %, respectively) it is lower by half than in the sample prepared without rinsing the magnesium hydroxide precipitate (CaO = 2.613 mass %).

As for the B₂O₃ content, Table 1 indicates that rinsing with alkalized distilled water of a high pH value (11.00 and especially 12.50) significantly reduces the quantity of boron adsorbed. When alkalized distilled water with pH = 11.00 is used to rinse the magnesium hydroxide precipitate, the B₂O₃ content in the final product – magnesium oxide- amounts to 0.1198 mass %, which is 38.15 % less than the B₂O₃ content in the sample prepared without rinsing of the magnesium hydroxide precipitate (B₂O₃ = 0.1937 mass %). If alkalized distilled water with pH = 12.50 is used to rinse the magnesium hydroxide precipitate, the B₂O₃ content is 0.0518 mass % which is 73.26 % less than the B₂O₃ content in the sample prepared without rinsing the magnesium hydroxide precipitate.

Therefore, we can conclude that the B₂O₃ content is significantly lower in the sample prepared with the rinsing agent with high pH values, especially pH = 12.50.

The increase in the pH value of the rinsing agent evidently reduces the content of boron adsorbed onto the magnesium hydroxide precipitate.

The increase in the pH value of the rinsing agent acts in direction of fuller dissociation of orthoborate acid, i.e. increased concentration of ions of higher dissociation degree, which are adsorbed more strongly. Graphical representations in Fig.1. and Fig.2. indicate that at pH = 12.50 the H₂BO₃^- concentration is 0.85 mol dm^-3, the HBO₃^2- concentration is 0.49 mol dm^-3, and the BO₃^3- concentration is 2.45^.10^-2; namely, H₃BO₃ has dissociated to 99.94 %, H₂BO₃^- to 36.27 %, and HBO₃^2- to only 4.82 %.

Thus, at pH = 12.50 dissociation in the first degree is complete, while only a third of the second degree dissociation has taken place, and the B₂O₃ concentration is very low. If the process is conducted like this, there is the possibility of adsorption of ionic boron forms, but it does not take place. Namely, the highly alkaline medium (pH = 11.00, and especially pH = 12.50) in rinsing the magnesium hydroxide precipitate by alkalized distilled water primarily affects the preferred adsorption of OH^- ions. So, due to alkaline medium, adsorption of ionic forms of boron is significantly lower. However, increased pH affects increased adsorption of Ca²⁺ ions on the Mg(OH)₂ precipitate in a strongly alkaline medium, i.e. at pH 12-13, but not to a greater degree.

4. CONCLUSION

- It has been established that the increase in the pH value of the rinsing agent reduces the content of boron (B₂O₃) adsorbed onto the precipitate of magnesium hydroxide from seawater

- Magnesium oxide samples obtained by rinsing the magnesium hydroxide precipitate with alkalized distilled water, pH = 12.50, contain approx. 70% B₂O₃ less than the samples prepared by rinsing the magnesium hydroxide precipitate with non-alkalized distilled water.

- The increase in the pH value of the rinsing agent increases the mass % of CaO, but not to a greater degree

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